Polymers are used in diverse applications from disposable plastic bottles to housing and from aerospace and automobile parts to packaging. Most of these plastics are made using petroleum as raw material (R. Bhardwaj et al., Biomacromolecules, 2006, 7, 2044-2051; M. S. Huda et al., Journal of Materials Science, 2005, 40, 4221-4229; and W. J. Liu et al., Polymer, 2005, 46, 2710-2721). However, petroleum, a fossil resource, is finite and by some estimates we are consuming petroleum at 100,000 times the rate at which the earth can produce (E. S. Stevens, Biocycle, 2003, 44, 24-27; and E. S. Stevens, Biocycle, 2002, 43, 42-45). In addition, composites, where two dissimilar constituents are combined cannot be recycled or reused easily and about 95% of them end up in landfills at the end of their life. As a result, increased attention has been paid in the past decade to developing polymers and composites using renewable resources. Other factors contributing to this ‘Sustainability Drive’ are the abundant availability of the biomass at relatively low cost and significant environmental benefits including zero or low carbon footprint (A. N. Netravali and S. Chabba, Materials Today, 2003, 22-29; and N. Supanchaiyamat et al., Green Chemistry, 2012, 14).
Soy protein is commercially available as defatted soy flour (SF), soy protein concentrate (SPC) and soy protein isolate (SPI). Soy protein is a long chain molecule (polymer) consisting of 18 different polar and nonpolar amino acids. Polar amino acids such as cysteine, arginine, lysine, histidine and others can be used to crosslink the protein to improve mechanical, thermal and physical properties as well as reduce water sensitivity and hydrophilicity (S. Chabba and A. N. Netravali, Journal of Material Science, 2005, 40, 6263-6273; and J. T. Kim and A. N. Netravali, Journal of Agricultural and Food Chemistry, 2010, 58, 5400-5407). Such crosslinked soy protein can be used as ‘green’ biopolymer (resin) as replacement for currently used petroleum based polymers (A. Gonzalez et al., Journal of Food Engineering, 2011, 106, 331-338; X. Huang and A. N. Netravali, Biomacromolecules, 2006, 7, 2783-2789; S. K. Lingamoorthy, Master of Science, Cornell University, 2010; P. Lodha and A. N. Netravali, Polymer composites, 2005 26, 647-659; X. Huang and A. Netravali, Compos Sci Technol, 2009, 69, 1009-1015; X. Huang and A. N. Netravali, Composites Science and Technology, 2009, 69, 1009-1025; X. Huang and A. N. Netravali, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 2008, 45, 899-906; X. S. Huang and A. Netravali, Compos Sci Technol, 2007, 67, 2005-2014; and R. Nakamura, A. N. Netravali, et al., Fire and Materials, 2013, 37, 75-90). Another advantage of the soy protein based resin, besides being green, is that it is inherently fire resistant and can perform better than some petroleum based resins (R. Nakamura, A. N. Netravali, et al., Fire and Materials, 2013, 37, 75-90).
It has been reported that soy proteins can be crosslinked using aldehydes such as formaldehyde, glutaraldehyde (GA), glyoxal and glyceraldehyde through Maillard reactions (S. B. M. Yasir et al., Food Chemistry, 2007, 104, 1502-1508). Park et al. (Journal of the American Oil Chemists' Society, 2000, 77) crosslinked SPI with GA to produce biopolymers with enhanced mechanical properties. Chabba et al. (Green Chemistry, 2005, 7, 576-581) crosslinked SF with GA which also led to an increase in Young's modulus with crosslinking Huang and Netravali (Compos Sci Technol, 2009, 69, 1009-1015) have reported the use of (3-isocyanatopropyl) triethoxysilane (ITES) for crosslinking of SPC. However, in moving towards ‘sustainable’ products, clean syntheses, use of natural renewable reagents and water based processing become highly desirable.
Nanofiber based membranes have been used in myriad of applications (G. Kim et al., J. Biomed. Mater. Res. Part. B Appl. Biomater., 2007, 81, 104-110; J. S. Im et al., Carbon, 2010, 48, 2573-2581; J. Y. Lee et al., Biomaterials, 2009, 30, 4325-4335; M. Alcoutlabi et al., J. Mater. Sci., 2013, 48, 2690-2700; W. Zhang et al., Appl. Phys. Lett., 2009, 95, 043304/1-043304/3; J. Fang et al., J., Chin. Sci. Bull., 2008, 53, 2265-2286; and D. F. Emerich et al., Curr. Nanosci., 2005, 1, 177-188). Almost all currently used membranes are made using non-biodegradable polymers derived from petroleum (S. Y Gu et al., Synth. Met., 2005, 155, 157-161; Z. Ma et al., Biomaterials, 2005, 26, 2527-36; L. Ji et al., Nanotechnology, 2009, 20, 1-7; A. L. Martinez-Hernandez et al., Curr. Nanosci., 2010, 6, 12-39; D. G. Yu et al., Nanotechnology, 2009, 20, 055104/1-055104/9; H. Wu et al., NanoLetters, 2010, 10, 4242-4248; and H. Niu et al., Carbon, 2011, 49, 2380-2388). For such non-biodegradable materials, there are no environmentally acceptable end-of-life solutions as of now. Most of them, unfortunately, end up in landfills. Availability of environment-friendly, biodegradable and fully sustainable plant derived polymers such as proteins, starches and cellulose have slowly begun to change this scenario. Plant derived proteins and starches also tend to be inexpensive compared to petroleum based polymers. Other factors contributing to the current ‘Green Movement’ are the abundant availability of the biomass and the possibilities of water based ‘green’ processing. These advantages have also resulted in developing ‘green’ nanofiber based membranes as replacement for petroleum derived non-degradable ones that are currently being used (S. J. Lee et al., J. Biomed. Mater. Res. A, 2007, 83, 999-1008; S. Agarwal et al., Polymers, 2008, 49, 5603-5621; B. Y. Gui et al., J. Biomed. Mater. Res. A, 2010, 93, 158-163; S. A. Sell et al., Polymers, 2010, 2, 522-553; L. Malinova et al., Funct. Mater. Lett., 2011, 4, 365-368; and J. Schiffman et al., Polym. Rev., 2008, 48, 317-352). Unlike petroleum based materials, most plant based materials can be easily composted after their intended life without harming the nature.
There is great interest in developing green nanofiber membranes. Several papers describe production of nanofiber membranes prepared by electrospinning of soy protein blends with polyvinyl alcohol (PVA), polylactid acid (PLA), zein or polyethylene oxide (PEO) (C. Yao et al., J. Appl. Polym. Sci., 2007, 30, 380-385; M. Phiriyawirut et al., Adv. Mater. Res., 2008, 55-57, 733-736; A. C. Vega-Lugo et al., Food Res. Int., 2009, 42, 933-940; and D. Cho et al., Polymer Degradation and Stability, 2012, 97, 747-754). However, disintegration of the nanofiber membranes in water is one of their biggest disadvantages.
This limitation can, however, be overcome by crosslinking of the polymer. Crosslinking has been the most commonly used technique to improve resistance to water as well as to improve the physical properties of polymers. Crosslinking is the process of chemically joining two or more molecules by a covalent bond. Crosslinking agents (or crosslinkers) are molecules that contain two or more reactive groups capable of chemically reacting with specific functional groups on proteins or other molecules. Despite the complexity of protein structure that contains several different amino acids, only a small number of protein functional groups comprise selectable targets for practical bioconjugation method (S. S. Wong et al., National Institutes of Health; CRC Press, USA, 1991). In fact, just four protein functional-group targets account for most of the crosslinking modifications. These include: (a) primary amines (—NH2): this group exists at the N-terminus of each polypeptide chain and in the side chain of lysine and arginine residues; (b) carboxyls (—COOH): this group exists at the C-terminus of each polypeptide chain and in the side chains of aspartic acid and glutamic acid; (c) hydroxyl (—OH); and (d) sulfhydryls (—SH): this group exists in the side chain of cysteine. For each of these protein functional groups there exist one to several types of reactive groups that are capable of reacting with them and have been used as the basis for synthesizing crosslinking reagents (S. S. Wong et al., National Institutes of Health; CRC Press, USA, 1991). Most commonly used crosslinkers for amine groups are bi-functional compounds, such as glutaraldehyde and glyoxal. Several papers have described crosslinking reactions in protein based resins with glutaraldehyde or glyoxal (S. K. Park et al., JAOCS, 2000, 77, 879-884; and S. Chabba et al., J. Mater. Sci., 2005, 40, 6275-6282). Both of these crosslinkers, however, are toxic and inappropriate from the environmentally-friendly point of view, and hence green crosslinkers are preferred.
The present invention is directed to overcoming these and other deficiencies in the art.